1. Field of the Invention
The present invention relates to a non-insulated type DC-DC converter circuit, and more particularly it pertains to a self-oscillation type DC-DC converter which is adapted to be of the step-down type and capable of generating self-oscillation by using a choke coil which comprises a single-winding inductance element and includes no feedback winding for self-oscillation.
2. Description of the Prior Art
Such a self-oscillation type DC-DC converter has been used as a relatively inexpensive power source because of the fact that the circuit arrangement thereof is simple and comprises only a small number of circuit elements. Referring to FIG. 8 of the accompanying drawings, there is shown a well-known self-oscillation type DC-DC converter circuit.
In the circuit arrangement of FIG. 8, a PNP switching transistor Q1 has its emitter connected to an input terminal 1 and its collector coupled to an output terminal 2 through a main winding N1 of a choke coil T. The emitter of the switching transistor Q1 is also connected to the drain of a field-effect transistor Q4 which has its source connected to its own gate and the cathode of a Zener diode DZ2 which has its anode grounded. The base of the switching transistor Q1 is connected to the collector of an NPN transistor Q5 and one end of a resistor R3.
The base of the transistor Q5 is connected to the cathode of the Zener diode DZ2, and the emitter of the transistor Q5 being grounded through a resistor R10. The other end of the resistor R3 is coupled to one end of a feedback winding N2 of the choke coil T through a capacitor C3. The other end of the feedback winding N2 is connected to the emitter of the switching transistor Q1.
A feedback circuit 4 is constituted by a serial circuit of the resistor R3 and capacitor C3. Further, the arrangement is made such that the end of the main winding N1 of the choke coil T which is connected to the collector of the switching transistor Q1 is of the same polarity as that end of the feedback winding N2 thereof which is connected to the emitter of the switching transistor Q1. A diode D1 is connected between the collector of the switching transistor Q1 and the ground, with the anode thereof grounded. Connected between the output terminal 2 and the ground are a smoothing capacitor C2, and a serial circuit of resistors R8 and R9 in parallel with the smoothing capacitor C2.
An NPN transistor Q6 has the collector thereof connected to the emitter of the switching transistor Q1, the emitter thereof coupled to the emitter of the transistor Q5, and the base thereof tied to the connection point between the resistors R8 and R9.
Description will now be made of the operation of the foregoing DC-DC converter circuit arrangement, with reference to FIG. 9 illustrating current and voltage waveforms which occur at various points in the circuit arrangement.
In FIG. 9, the reference symbols are as follows:
V.sub.IN Voltage applied to the input terminal 1; PA1 V.sub.BE1 Bias voltage applied between the base and the emitter of the switching transistor Q1; PA1 V.sub.CE1 Voltage occurring between the collector and the emitter of the switching transistor Q1; PA1 I.sub.B1 Base current of the switching transistor Q1; PA1 I.sub.N1 Current which flows from collector of the switching transistor Q1 to the output terminal 2 through the main winding N1 of the choke coil T; PA1 E.sub.N2 Terminal voltage occurring at the feedback winding N2 of the choke coil T and which is positive when the capacitor C3 is at a higher potential; PA1 V.sub.OUT Voltage derived from the output terminal 2; PA1 V.sub.B6 Voltage applied to the base of the transistor Q6
voltage V.sub.IN is applied to the input terminal 1 at a time point t.sub.0, then the transistor Q4 is rendered operative, and thus the base of the transistor Q5 is positively biased so that the transistor Q5 is turned on. Thereupon, forward base current I.sub.B1 begins to flow in the base of the switching transistor Q1 so that the switching transistor Q1 is turned on.
During time period from the time point t.sub.0 to t.sub.1, the switching transistor Q1 remains turned on so that the current I.sub.N1 which increases linearly with time is caused to flow through the main winding N1 of the choke coil T. This current I.sub.N1 is charged at the smoothing capacitor C2 so as to increase output voltage V.sub.OUT occurring at the output terminal 2. The increase in the current I.sub.N1 results in negative voltage E.sub.N2 being generated at the feedback winding N2 of the choke coil T.
The base current I.sub.B1 of the switching transistor Q1 is also caused to flow in the feedback circuit 4 because of the voltage E.sub.N2, and becomes equal to a current value resulting from a combination of the current I.sub.N2 flowing through the feedback winding N2 and the collector current I.sub.C5 of the transistor Q5.
The current I.sub.C5 is caused to decrease gradually due to the fact that the increase in the output voltage V.sub.OUT results in collector current I.sub.C6 of transistor Q6 being increased. Thus, there is a tendency that the current I.sub.B1 decreases during the time period from the time point t.sub.0 to the time point t.sub.1.
Immediately prior to the time point t.sub.1, the current I.sub.N1 flowing through the collector of the switching transistor Q1 and the main winding N1 of the choke coil T is increased up to and saturated at a level equal to the forward base current I.sub.B1 multiplied by the current amplification factor h.sub.EF1 of the first switching transistor Q1, i.e., (h.sub.FE1 .times.I.sub.B1).
When the current I.sub.N1 becomes saturated and thus no current variations occur, the voltage E.sub.N2 generated at the feedback winding N2 of the choke coil T is deprived of electromotive force, and the base current I.sub.B1 of the switching transistor Q1 is decreased by the amount of the proportion of the current which was caused to flow in the feedback circuit 4 because of the voltage E.sub.N2. Obviously, the decrease in the base current I.sub.B1 decreases the current I.sub.N1 flowing through the collector of the switching transistor Q1 and the main winding N1 of the choke coil T.
The decrease in the current I.sub.N1 causes the counterelectromotive force E.sub.N2 to be generated in the feedback winding N2, which assumes a positive voltage value as shown in FIG. 9. The voltage E.sub.N2 having the positive voltage value provides a reverse bias between the base and the emitter of the switching transistor Q1 through the feedback circuit 4, thus causing the base current I.sub.B1 to further decreased, while at the same time the current I.sub.N1 is decreased.
The decrease in the base current I.sub.B1, the decrease in the current I.sub.N1, and the generation of the voltage E.sub.N2 are caused to occur in the form of chain reaction, so that the switching transistor Q1 is turned off rapidly.
Charges accumulated at the base region of the switching transistor Q1 when the latter is conducting, are discharged due to the voltage E.sub.N2 so that spike-like base current I.sub.B1 is caused from the base toward the emitter of the switching transistor Q1.
The discharging of the charges accumulated at the base region causes the switching transistor Q1 to be completely turned off at the time point t.sub.1. The operation after the current I.sub.N1 becomes saturated until the switching transistor Q1 is turned off, is performed in a very short time; thus it can be considered that the saturation of the current I.sub.N1 and the turning-off of the switching transistor Q1 are effected substantially at the same time.
During the time period from the time point t.sub.1 to a time point t.sub.2 in FIG. 9, even after the switching transistor Q1 is turned off, the current I.sub.N1 resulting from counterelectromotive force is permitted to continue flowing in the main winding N1 of the choke coil T through the diode D1 and decreases linearly with time. As the current I.sub.N1 is decreased, the output voltage V.sub.OUT, voltage V.sub.B6, and current I.sub.C6 are decreased.
At this point, the counterelectromotive voltage E.sub.N2 provides a reverse bias between the base and the emitter of the switching transistor Q1, while at the same time, capacitor C3 of the feedback circuit 4 is charged by current I.sub.N2 which is caused to flow in the feedback winding N2 because of the counterelectromotive voltage E.sub.N2.
At the time point t.sub.2, the charging of the capacitor C3 is completed, and thereupon, the voltage providing the reverse bias between the base and the emitter of the switching transistor Q1 is eliminated. At this point, the base of the transistor Q5 is always in a positively biased state, so that the switching transistor Q1 is turned of again.
During time period from t.sub.2 to time point t.sub.3 when the switching transistor Q1 is turned on, operation similar to that performed during the time period from the time point t.sub.0 when the voltage V.sub.IN was applied to the time point t.sub.1 when the switching transistor Q1 is turned off, is performed, and the time point t.sub.3 when the switching transistor Q1 is again turned off, is reached.
The above-described operation is repeated so that self-oscillation is generated.
Description will next be made of operation for stabilizing the output voltage of the conventional DC-DC converter shown in FIG. 8.
As a result of the self-oscillation, the output voltage V.sub.OUT builds up and voltage V.sub.B6 also builds up, so that transistor Q6 is positively biased at the base thereof and thereby turned on. Differential operation is performed by the transistors Q5 and Q6; thus, when the transistor Q6 is turned on and current I.sub.C6 is caused to flow in the collector thereof, the collector current I.sub.C5 of the transistor Q5 is decreased.
If the output voltage V.sub.OUT exceeds a predetermined level, then the current I.sub.C5 is decreased, and the current I.sub.B1 is also decreased. Thus, since the current I.sub.N1 is of a low value and flows for only a short time, operation for restraining the magnitude of the output voltage is performed.
On the other hand, if the output voltage V.sub.OUT becomes lower than the predetermined level, then the collector current I.sub.C5 of the transistor Q5, i.e., the base current I.sub.B1 of the switching transistor Q1 is increased.
Immediately after the current I.sub.N1 flowing in the main winding N1 has become equal to (h.sub.FE1 .times.I.sub.B1), the switching transistor Q1 is turned off; thus, if the current I.sub.B1 is high, the current I.sub.N1 is permitted to continue flowing until it becomes high, thereby increasing the magnitude of the output voltage V.sub.OUT.
As mentioned above, self-oscillation type DC-DC converter is mainly employed as low output capacity, low cost power source.
The circuit of FIG. 8 is capable of providing self-oscillation and constant voltage output by virtue of its simplified circuit arrangement, and arranged such that the choke coil includes, besides the main winding, the feedback winding for achieving oscillating operation. Hence, the conventional circuit arrangement is disadvantageous in that it is required that the plural windings be used even if the number of output is single (one channel), thus increasing the cost and size of the power source.